Giant impact theory

. According to the giant impact hypothesis, a massive projectile (A), about the size of Mars, struck the young, still-forming Earth (B) in a catastrophic, glancing blow nearly 4.6 billion years ago, resulting in a tremendous explosion and the jetting outward of both projectile and proto-Earth mass. Some fraction of this mass remained in Earth orbit (C), while the rest escaped Earth or impacted again on Earth's surface. A proto-Moon began to form from the orbiting material (D), accreting neighboring matter, and finally became the Moon (E). It may be mostly derived from the crust and mantle of the Earth and/or the impacting object, accounting for the Moon's relatively low mean mass density and lack of iron when compared with the Earth. The Moon accumulated so rapidly that the outer crust was molten, helping to account for the relative lack of water and other volatile elements. Then, as the crust cooled, the newborn Moon swept up the remaining objects nearby, blasting out impact basins and pockmarking the surface with numerous craters. (Courtesy of Alan P. Boss, Carnegie Institution of Washington.)